VARIED IMPACT OF FAULTS AND FRACTURES ON PETROLEUM MIGRATION AND PRODUCTION IN THE PISMO-HUASNA SYNCLINE, CENTRAL CALIFORNIA

The Pismo-Huasna Basin, California, is a petroliferous Neogene basin located in a fold and thrust belt between the Coastal Ranges and the Western Transverse Ranges. The Obispo, Monterey, and Pismo formations within the Pismo- Huasna Syncline deformed in a variety of styles and intensities due to their distinct lithologies and the typical mechanical layer thicknesses of primary bedding. Structures play a crucial role in the migration and production of hydrocarbons by acting as conduits or barriers. The Obispo, Monterey, and lower Pismo formations exhibit strain as fractures, faults, and injectites that act as conduits and form in various orientations. In contrast, the Edna Member of the Pismo Formation of the Arroyo Grande oil field contains abundant deformation bands, many of which exhibit fault-sealing characteristics. Some of the deformation bands act as permeability barriers that hinder the downward migration of oil from uplifted and exposed reservoir rocks and likely compartmentalize the subsurface reservoir. We collected samples from the deformation bands and undeformed rocks to evaluate the lithologic characteristics that influence fluid migration in the Edna Member. During thin section creation, solvent extraction was used to dissolve the oil present to allow for epoxy impregnation. Despite multiple rounds of extraction, the deformation bands still contain oil, whereas the sand outside the bands, previously saturated with oil, are now clean. This implies that the permeability of the deformation bands is negligible. Quantitative image analysis of the average grain size between the deformation band and host rock indicates there is up to a 41% decrease in average grain size due to cataclasis. Porosity within the deformation bands is reduced to 4-17% from 22-32% in the host rock. Orientations of deformation bands are the same on both limbs of a syncline, suggesting the bands formed after folding. These initial data suggest that the deformation bands formed after the charging of Edna Member with hydrocarbons, and after folding during the most recent convergent regime. The relative timing of formation of the deformation bands suggest they didn’t influence the secondary migration of hydrocarbons, but is likely influencing the efficiency of hydrocarbon production due to compartmentalization of the reservoir.